The invention lies in the field of manufacturing disc-like plastic objects by means of a mould. The manufacture of disc-like objects by means of a mould is known. Diverse known techniques are, among others, injection moulding, injection pressing, injection compression and coining. The invention is applicable to all of these as well as other suitable techniques.
The invention relates to a mould for manufacturing disc-like objects, which mould comprises:
at least two mould parts which are mutually movable between a closed position in which they bound a mould cavity, which mould cavity is connected to an injection conduit for admitting heated, plasticized plastic under pressure into said mould cavity, and an opened position in which a disc-like object can be removed; and PA1 removing means for removing a central portion from a disc-like object to form a central hole present therein, which removing means comprise a cylindrical part which is movable in axial direction in the closed situation of the mould cavity. PA1 injection bush, punch: about 10.degree. C.-30.degree. C. PA1 mirrors (plane surfaces of the mould cavity): about 40.degree. C.-70.degree. C. PA1 peripheral zone: about 50.degree. C.-100.degree. C. The respective tempering means herefor specified in the claims can, subject to the wish to adjust specific temperatures, be adjustable either in combination or indeed individually.
Such a mould is known generally and in many embodiments.
The disc-like objects are in particular transparent information carriers such as CDs and DVDs. For a correct scanning of the digital information arranged thereon, such information carriers must comply with very specific technical requirements. In addition to flatness and transparency, such technical requirements also include properties relating to birefringence, which must meet a required standard within close tolerances and must generally be as low as possible.
The birefringence is an optical measurement value for internal material stresses. These internal stresses are introduced during manufacture by injection moulding or injection pressing of polycarbonate substrates. They are caused by shrinkage, flow lines and/or inclusions of air or other contaminants. The stresses result in the light of the laser being additionally diffused during scanning, whereby phase differences occur when a substrate is scanned. This diffusion of light rays results in a decrease in the effective intensity of the laser beam. Thereafter, light rays are also deflected and diffusion occurs when radiation transfers from the one medium to another medium with a different density.
The wavelength of a scanning laser is roughly 780 nm in air and about 500 nm in polycarbonate. The light diffusion index is 1.55. In the specification for the birefringence of polycarbonate substrates is stated that this may vary by a maximum of 20% from the given value and may thus amount to a maximum of 100 nm. A larger variation will certainly result in problems. It is remarked here that in the case of production processes for mass-produced articles such as those under consideration, it must be ensured under all circumstances that the relevant norm is achieved, even in the most unfavourable conditions. It is therefore desirable to design a mould such that the birefringence is thus as low as possible in all conditions and is nominally considerably lower than the stated tolerance limits.
The birefringence is measured in terms of phase differences and decrease in light intensity between a .chi.-component and the .gamma.-component of polarized light. The birefringence is expressed in nanometres (nm).
A further explanation concerning the described concepts can be found in "Kunststoffe German Platics" 1989/1, reprint, "Birefringence in Optical Information Storage Discs made of Polycarbonate".
For manufacture of information carriers of the described type an optionally replaceable wall of the mould cavity is provided with a negative impression of information to be arranged on the disc-like object. Such information must be transferred with very great reliability and accuracy onto the disc-like object.
It is further necessary to prevent the occurrence of a phenomenon known as "dishing". This is the phenomenon where a disc-like object is not flat but has a shape which curves in substantially rotation symmetrical manner. In this same context the phenomenon of "warpage" often occurs, wherein the disc-like object acquires a saddle shape. It may even occur that a disc-like object produces so-called "click-clack". This is the phenomenon where an object has two mutually differing stable shapes which can transpose into each other by exerting a force on the central zone.
It is an object of the invention to essentially prevent to a considerable extent the described undesired phenomena and generally improve the quality of a disc-like object.
It a further object of the invention to embody a mould of the described type such that the time required to manufacture a disc-like object is reduced.
Summarizing very briefly the objectives of the invention as specified above, a main purpose of the invention is to improve the quality of a manufactured disc-like object while reducing the cycle time.
With a view to these objectives the invention aims to considerably increase the "operating window". This operating window is understood to mean the total range in which combinations of parameter values are possible wherein the set requirements can be fulfilled.